IHEP–HD/96–6 Baryon Production in ALEPH Presented on behalf of the ALEPH Collaboration by U. Becker 1 Institut f¨ur Hochenergiephysik der Universit¨at Heidelberg Heidelberg, Germany Abstract Several recent results of the ALEPH Collaboration covering different aspects of baryon production on the Z resonance are presented. In particular production rates of hyperons, the full kinematical reconstruction of the Λb, observation of Ξb in its semileptonic decay, and the measurements of the polarization of Λ and Λb baryons are discussed. 1Talk given at the HADRONS 96 conference 9.6.– 16.6. 1996 in NOVY SVET, Ukraine 1 1 Introduction Baryon production in quark fragmentation is still poorly understood. The existing models reproduce the available data quite well. To distinguish between these models and to come to a more fundamental understanding of the production mechanism of baryons more experimental input is needed. Results of the ALEPH Collaboration on hyperon and b – baryon production in Z decays are reported here. Whenever possible, not only the overall production rates but also the momentum spectra of baryons have been investigated and compared to Monte Carlo predictions. While the number of produced hyperons is quite large the production rate of b – baryons is low. Only few of them have been observed by LEP experiments so far. The full recon- struction and mass determination of the Λb and the observation of the Ξb through Ξ – lepton correlations will be described. Z decays provide many possibilities to test the Standard Model. Quarks produced in Z decays are expected to obtain a strong longitudinal polarization due to parity violation in the electroweak interaction. While mesons always cascade down to spin zero pseudoscalar states retaining no polarization information, baryons might preserve a large fraction of the initial quark polarization. Recent measurements of the Λ and Λb polarizations are presented here. The data sample used for these studies has been taken with the detector ALEPH at the LEP storage ring at a center of mass energy of about 91 GeV. The detector and its performance are described in detail elsewhere [1]. 2 Hyperon production ALEPH has studied recently the inclusive production rates of several octet and decuplet hyperons: Σ±(1385), Ξ−,Ξ0(1530) and Ω−[2]. Except for the Ω− also the energy spectra have been measured. The four hyperons have been reconstructed in their decays: Σ±(1385) → Λπ± Ξ− → Λπ− Ξ0(1530) → Ξ−π+ Ω− → ΛK− where the Λ candidates are tagged in the decay channel Λ → pπ−. The resulting mass distributions are shown in Fig. 1a - d. Except for the Ω− (due to low statistics) the energy spectra have been determined and compared with Monte Carlo predictions from JETSET 7.3[6] and UCLA[7] (Fig.2). The agreement is quite reasonable with slight preferences for JETSET. The mean multiplicities per Z decay have then been determined using JETSET to extrapolate into the inaccessible low energy region. 2 Figure 1: (a)The signal for Ξ− → Λπ−. (b) The signal for Ω → ΛK−. (c) The mass spectrum for Σ±(1385) → Λπ±, fit to a background shape, a Ξ− contribution, plus a Breit – Wigner resonance. Also shown is the background – subtracted spectrum. (d) The mass – difference spectrum for Ξ0(1530) → Ξ−π+ candidates, fit to the background shape plus a Breit – Wigner resonance convolved with a gaussian. The preliminary results are: hNΞ− i + hNΞ¯+ i =0.0285 ± 0.0007 ± 0.0020 hNΞ0(1530)i + hNΞ¯0(1530)0i =0.0072 ± 0.0004 ± 0.0006 hNΣ±(1385)i + hNΣ(1385)¯ ±i =0.065 ± 0.004 ± 0.009 hNΩ− i + hNΩ¯ + i =0.0010 ± 0.0002 ± 0.0002 While our measurement of the Σ±(1385) rate is in good agreement with the Monte Carlo prediction it is nevertheless about 1.7 times higher than the corresponding measurements from DELPHI and OPAL[3],[4],[5]. Fig.3 gives an overview about the hyperon production rates at LEP[8]. 3 ± − 0 Figure 2: The measured xE distribution for Σ (1385), Ξ ,andΞ(1530) (with antiparticles included), compared with predictions from the UCLA and JETSET 7.3 models. ALEPH OPAL Λ DELPHI Σ L3 Σ✳ Ξ /(2I+1)) per event Ω Baryon JETSET 7.4 PS (N ✳ HERWIG 5.8 Ξ UCLA 7.44 Hyperon Mass (GeV/c2) Figure 3: Production rates of hyperons at LEP together with predictions of JETSET, HER- WIG and UCLA models. Lines are drawn to guide the eye. 4 3b–baryons The situation in the sector of heavy baryons containing a b – quark is quite different. Due to their low production rates combined with small branching ratios (in the order of a few %) and long decay chains only very few b – baryons have been observed at LEP so far, namely ∗ the Λb,Ξb (which will be presented here) and Σb,Σb by the DELPHI Collaboration [9]. 3.1 The Λb − + − The Λb has been observed indirectly since several years in Λl and Λc l correlations (where l = muon or electron). Its production rate has been determined by e.g. ALEPH to be + −3 Br(b → Λb) · Br(Λb → Λc lνX¯ )=(0.76 ± 0.15 ± 0.13)10 [10] but its mass remained only + − poorly known. Now the Λb has been fully reconstructed in its decay channel Λb → Λc π + ¯ 0 with Λc → pKπ, pK or Λπππ. Several cuts have been applied to reduce the background. The result is shown in Fig.4 The background has been estimated to be 0.38 ± 0.05 entries leaving us with a statisti- cal significance of the observed peak at the 3.3 σ level, not taking into account the mass clustering of the four events. The Λb mass has been found to be (5614 ± 21(stat.) ± 4(sys.))MeV/c2[11]. This is in good agreement with the recent measurements of DEL- PHI (5668 ± 16 ± 8)MeV/c2[12] and CDF (5623 ± 5 ± 4)MeV/c2[13] and the PDG94 value (5641 ± 50)MeV/c2[14]. Figure 4: (a) Λcπ invariant mass distributions for the right – sign combination and (b) for the wrong – sign combinations. (c) Λb invariant masses for the four selected candidates. The dotted lines indicate the ±1σ values around the ALEPH average measurement. 5 3.2 The Ξb A search for Ξ−l− correlations (again l stands for muon or electron) in the same hemisphere of the detector has been made to find evidence for Ξb production. After correcting for other − − possible sources of Ξ l pairs as e.g. Λb B or accidentals an excess of 22.5 ± 5.7 like – sign combinations with respect to opposite – sign combinations has been observed. It is therefore interpreted as from the semileptonic decay of the Ξb and can be translated into the following product of branching fractions: − − −4 Br(b → Ξb) · Br(Ξb → Ξ l νX¯ )=(5.3±1.3±0.7)10 per lepton species[15]. This value is in good agreement with the DELPHI result[16]. 4 Polarization Polarization of the primary quarks coming from Z decays is due to the parity violating inequality of the Z coupling to right and left handed fermions. Ignoring mass effects and gluon radiation (which may depolarize the quarks by about 3% [17]) the expected polarization can be written as q −2gvga PL = 2 2 gV + gA where gv and ga are the vector and axial vector coupling constants of the Z to the quark. 2 With sin ϑW = 0.23 a polarization of −94% for downtype quarks is expected according to the Standard Model, but may be diluted substantially during the hadronization phase due to spin – spin forces. Mesons always cascade down to spin zero pseudoscalar particles which do not retain any polarization information. In contrast baryons are expected to preserve a sizeable fraction of the initial quark polarization which, however, gets smaller if the weakly decaying ground state baryon is produced in the decay of a higher resonance[18],[19]. Therefore the mean measureable polarization will depend on the production rate of these resonances and will thus give additional information about the hadronization phase. 4.1 The Λ polarization The polarization of Λ hyperons can be measured by investigating the decay angle ϑ∗ in their decay Λ → pπ−.Hereϑ∗is defined as the angle between the proton momentum in the Λ restframe and the Λ boost direction. The ϑ∗ distribution is expected to follow ∗ Λ ∗ R(ϑ )=1+αPL cos ϑ with the asymmetry parameter α =0.642 ± 0.013 [14] and the Λ Λ polarization PL . The observed Λ polarization is diluted with respect to the primary s – quark polarization by those Λ baryons which do not contain a primary quark. To derive Λ a Monte Carlo prediction of PL , first the fraction of Λ’s containing a primary quark has 6 been determined in the data by measuring the Λ forward – backward asymmetry and the correlation of back – to – back ΛΛ¯ pairs. The differences between data and MC have been used as correction factors for all possible sources of polarized Λ’s (see [21] for details). Then the fraction of Λ baryons coming from e.g. Λb, Σ etc. has been corrected, and the expected polarizations [20] have been added up. The ALEPH measurement is shown in Fig.5 together with the expectation. For Λ’s with z = p(Λ)/PBeam > 0.3 the polarization Λ has been determined to be PL = −0.32 ± 0.07 [21], in good agreement with the JETSET prediction of −0.39 ± 0.08. 0.6 1.25 0.5 1 0.4 0.75 0.3 0.5 0.2 0.25 0.1 0 0 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 0 0.1 0.2 0.3 0.4 0.5 0.6 Figure 5: Right: Fit of the longitudinal Λ polarization to the decay angle distribution for z>0.3.